Comparison of ALINGAAS/GAAS Superlattice Photocathodes Having Low Conduction Band Offset Page: 2 of 5
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1. Interband absorption smearing 6 due to bandedge fluctuations;
2. Hole scattering between the HH and LH states that causes a broadening y of
the LH band;
3.Spin precession due to an effective magnetic field generated by the lack of
crystal inversion symmetry and spin orbit coupling;
4.Electron-hole scattering (negligible comparing to 3);
5. Less polarization selectivity in the BBR;
6. Scattering and trapping of electrons in the BBR.
The first two mechanisms are related to the HH-LH splitting for supporting
the spin selection rules. A systematic study on the GaAs/GaAsP structure 
showed that after a certain splitting level, no increase of polarization could be
obtained. Mechanisms 5 and 6 are related to the effects of the BBR and will be
independently studied in the future. Mechanisms 3and 4 are material related and
they take place during the transport of electrons in the photocathode active
region. In the GaAs/GaAsP SL, the electrons tunnel through high barriers in
order to reach the cathode surface. In order to lower the barriers in the CB
without lowering the barriers for the holes in the VB (to preserve the HH-LH
splitting), a quaternary alloy InGaAlAs/GaAs SL was designed and tested at St.
Petersburg University, and polarization as high as 91% was achieved with an
optimized structure . The measurements were repeated at SLAC on samples
grown by SVT Associates. The results are presented in this paper and they are
compared with simulations.
2. Design of Flat Conduction Band SL Structures
The model for the emitted electron polarization  indicates that polarization is
inversely proportional to the electron transit time in the active region. Motivated
by this concept, flat CB structures based on In.AlyGa1XyAs/GaAs strained
barrier SL were designed. The x (In) percentage lowers the bandgap, controls
the CB offset AEc and induces compressive strain in the barriers in order to
achieve the desirable HH-LH splitting. The y (Al) percentage controls the size
of the SL bandgap and preserves high barriers for the holes in the VB. The goal
is to design a structure with as flat a CB as possible, while maintaining a
substantial (>30meV) VB splitting. The CB gets anomalously flat for x=1.ly.
The VB splitting is determined by the induced strain in the barriers controlled
by the Indium percentage and the quantum confinement of the wells controlled
by the barrier/well sizes. For the Al0.21Ino.20Gao.59As/GaAs SL with 1.5nm wells
and 4nm barriers the HH-LH splitting is >50meV.
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Ioakeimidi, K,; Maruyama, T.; Clendenin, J.E.; Brachmann, A.; Garwin, E.L.; Kirby, R.E. et al. Comparison of ALINGAAS/GAAS Superlattice Photocathodes Having Low Conduction Band Offset, article, March 31, 2006; [Menlo Park, California]. (digital.library.unt.edu/ark:/67531/metadc876623/m1/2/: accessed October 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.